Reserved - edit

[arbitraryarmor]

The muffled room in which we dine
at tables doomed for eight or nine; I like the word "doomed" here personally, but I would probably choose a different one because it's pretty abstract
the champagne glasses deftly clinked,
though champagne isn’t what we drink. Nice contrast with the previous line
The chicken’s textured rubber duck
and vegetables are overcooked; I probably wouldn't use slant rhyme here.
rain dribbles down the window-panes; "Rain" breaks the meter; whether or not this is intentional is unknown to me
the walls exhibit aged stains. The meter's off on this one as well, although you could go classic English on us and pronounce it "ag'ed"
The waitress frowns absorbed, remote; Correct grammar would be "The waitress frowns, absorbed and remote", so I think this line needs some reworking from a grammar standpoint; at the very least, there should be a comma or a semicolon after 'frowns'.
you shape a sound, you clear your throat
then whisper something indistinct; This line offsets the tempo; the presence of the word "then" makes this line feel attached to the couplet previous which makes the next line feel stranded.
our stiltedness makes others wince. Seems like an overreaction; the audience can focus on the small details, but it makes no sense for the people in the restaurant to be. Also, who's "our"? Lastly, your lines go "...then whisper something indistinct; // our stiltedness makes others wince." which doesn't flow well in my head.
The portraits yawn, the music plinks; "Plinks" is a poor choice of words here because it doesn't rhyme; "skips" works thematically and sounds much better
a napkin’s drawn to bloodless lips.
So many seats remain unfilled
to celebrate how time is killed.
 

Overall I actually quite enjoyed the poem; the rhythm is quite strong and the good imagery paints the gray subject without being too down about it.